Beware of this type of apps. The Bing Health and Fitness lacks serious features in the way it calculates calories (missing height, weight and age). Plus, I've tried several of them with the same data (exercise, time spent, etc) and got very different results (we're talking hundreds of calories of difference between apps and websites).
So choose wisely and, maybe, check with your physician or nutritionnist (if you have one) to confirm the results.
You're never going to get an accurate reading on calories burned or the way it estimates your calories. You just need to find out how many calories you need, yourself, and use it as a tracker. I wouldn't even use their "guidelines" for Carbs, Protein and Fats. Make your own, with a little research.
I've learned how many calories I need to lose weight, maintain weight or gain weight, so I plug I my foods and go from there. Thats about all you can really get out of these Apps. They could also be useful for health related issue, trying to reduce or increase certain nutrients in your diet.
Most things that estimate calories burned for a specific activity are over estimating. Sometimes badly! Even cardio/gym machines. Everyone's metabolism is different, its impossible to guess.
But, yeah, MyFitnessPal barely works. It often doesn't at all.